Support for LimitPushDown options in query context and TimeFormatExtractionFunction bug fix

This release includes:

• Support for limitPushDown options in query context (http://druid.io/docs/latest/querying/groupbyquery.html#groupby-v2-configurations)
• Taking datetime pattern as String instead of SimpleDateFormat in TimeFormatExtractionFunction (Fixing issue #80 )

Contributor:

• Zubair (@moezubair )